This invention relates to improvements in power transmission systems of the belt type used for small sized, lightweight motorcycles and the like.
A small sized, lightweight motorcycle generally has a power transmission system of the belt type.
Such a power transmission system comprises a driving side variable diameter pully mounted on the output end of the crankshaft, a driven side variable diameter pulley mounted on a portion in proximity to the axle, and a V belt installed over said pulleys. In such a power transmission system, a gear change mechanism is provided on a portion in proximity to the axle, either on the upstream of the driven pulley to output power from the gear change mechanism to the axle or on the upstream of the driven pulley to take out power from the shaft of the driven pulley. In each case, the gear change mechanism having a multiplicity of gears and shafts is provided in the vicinity of the axle. However, in a small sized, lightweight motorcycle, a suspension mechanism of the power unit swing type where an engine, a crankcase and a transmission case are integrated is generally used in many cases. The transmission including said crankshaft is incorporated in the case and a gear change mechanism is provided about the rear wheel axle supported by the rear portion of the case.
In the foregoing conventional transmission mechanism, because a heavy gear change mechanism is mounted about the rear wheel axle, the proportion of the load on the rear cushion unit suspending the power unit becomes great. Because heavy objects are placed away from the swing pivot of the power unit, inertia about the pivot also becomes great.
Further, because the gear change mechanism is incorporated in the rear portion of the transmission case, it is located away from the crankcase from the viewpoint of lubrication, requiring separate means of lubrication, making piping, routing and other oil feeding structure complicated and requiring the transmission case to be made oiltight up to the rear portion. Manufacture of the transmission case becomes troublesome and complicated and requires sealing materials of high oiltight quality.